1. Field of the Invention
The present invention relates to the field of gas turbines. It refers to a gas turbine with a plurality of individual burners which are connected to a fuel distribution system and are supplied with fuel from a common fuel feed via the fuel distribution system, there being provided, within the fuel distribution system, means by which the fuel mass flows to the individual burners can be varied in order to improve the operating perimeters of the gas turbine.
Such a gas turbine is known, for example, from the publications U.S. Pat. No. 5,024,055 or U.S. Pat. No. 5,319,931 or U.S. Pat. No. 5,373,692.
2. Discussion of Background
High-powered gas turbines, such as are used in combined cycle power stations, are often operated with a plurality of burners. Examples of gas turbines of this kind are found in the publications initially mentioned. Gas turbines of this type which are used commercially have been sold by Asea Brown Boveri AG, for example under the type designations GT 13E2, GT 24 or GT 26. A block diagram of such a gas turbine is illustrated in FIG. 1. The gas turbine 10 includes a compressor 12 and a turbine 14, which are conventionally arranged on a common shaft 13. Between the compressor 12 and turbine 14 is provided a combustion chamber 18, in which a plurality of (in the example, six) burners B1, . . . , B6 generate hot combustion gases as a result of the combustion of a liquid or gaseous fuel. The burners B1, . . . , B6 may be equipped, for example, with extractable burner lances which introduce the fuel and, if appropriate, combustion air for injection into the burner. The various burners B1, . , B6 of the gas turbine 10 are supplied with fuel from a fuel source (not shown) via a fuel distribution system 21. The fuel is fed to the fuel distribution system 21 via a central fuel feed 28 and one or more fuel regulating valves 20. Individual branch lines 23 extend from the common distribution line 22 to the burners B1, . . . , B6. The combustion air necessary for the combustion of the fuel is sucked in by the compressor 12 via an air feed 11, compressed and subsequently fed, via a compressed-air duct 16, into a plenum chamber 19, from where it passes through corresponding orifices into the burners B1, . . . B6 or fuel lances. The hot combustion gases from the combustion chamber 18 pass, via a hot-gas duct 17, into the turbine 14, where they perform work in one or more stages and are subsequently available, at an exhaust-gas outlet 15 of the turbine 14, for further use (for example, for generating steam in a waste-heat boiler).
In order, then, to ensure that the (leanly operated) burners deviating downward from the average value remain well above the extinguishing limit LG, and so that the machine can be operated reliably and stably, the average value M to be set via the fuel regulating valve 20 must be selected well above the lean extinguishing limit LG. Fuel-enriched burners (B3, B4, B5) therefore run in addition to lean burners (B1, B2, B6). In an extreme situation, the fuel-enriched burners must even assist combustion in the lean burners (external pilot control). The result of this, ultimately, is that the emission average value M to be set in this way deviates markedly from the minimum obtainable (individual burner evidence in the individual burner test). Under some circumstances, therefore, in the case of a multiburner configuration, a considerably worsened lean extinguishing limit can be obtained, as compared with the individual burner. The same also applies to the temperature profile. The actual aim, specifically also to achieve a homogeneous emission and temperature profile, particularly in the case of annular combustion chambers, by means of homogeneous combustion, has hitherto, in practice, been achieved only approximately on account of manufacturing tolerances, design accuracy and the like. This may, indeed, be sufficient for a reliable (rough) setting of the gas turbine, however, the existing potential of gas turbines with multiple burner arrangements is certainly not fully utilized in this way.
U.S. Pat. No. 5,024,055 describes a gas turbine with a multiple burner arrangement, in which the unburned hydrocarbons (UHCs) of the individual burners are detected at the turbine outlet by means of sensors arranged in a distributed manner. The measured values are used for adjusting the fuel/air ratio for each individual burner to its optimum value via individual regulating valves in the fuel feed lines (branch lines) to the burners. Regulation of the air feed also takes place in addition. Although this regulation makes it possible for essentially optimum combustion ratios to be achieved permanently in the individual burners, this regulation, with its multiplicity of sensors and, in particular regulating valves (two regulating valves for each two-stage burner), is not only extremely complicated, but also susceptible to faults.
U.S. Pat. No. 5,319,931 proposes a method for trimming the fuel/air ratio of the individual burners in a gas turbine with a two-stage multiple burner arrangement and with an inhomogeneous air feed, in which a trimming unit, which contain s an adjustable valve, is arranged in each of the fuel feed lines (branch lines) to the burners. In order to compensate the unequal operating parameters of the burners, which occur as a result of an unequal air feed, the fuel feed to the individual burners is retrimhed manually or automatically by means of the trimming device in accordance with specific measurement values (for combustion chamber pressure, temperature and fuel mass flow). Trimming, in this case, relates to an unequal air feed, but not to an unequal fuel feed. Furthermore, in this case too, a multiplicity of valves equipped with moveable parts and susceptible to faults are used. In addition, during operation in the automatic mode, various diaphragms also have to be arranged in the trimming units, in order to ensure reliable and accurate regulation.
Finally, U.S. Pat. No. 5,373,692 discloses a trimming system for minimizing the NOx emissions of a gas turbine with a multiple burner arrangement, in which two parallel-connected regulating valves are arranged in each case in the fuel feed lines (branch lines) to the individual burners, said regulating valves being actuated by central control in accordance with the measurement values from an NOx sensor. The main valve of the parallel connection serves, in this case, for the basic setting, whilst the secondary valve is provided for trimming. In this case, too, due to the two valves for each burner, there is a considerable outlay in terms of apparatus and increased susceptibility to faults.